Porous cellulose photonic film via controlled unidirectional interlayer freezing for rapid visual sensing.

Structure color, arising from the interaction of light with regularly arranged sub-micrometer-sized structures, has spurred interest in sensor design. However, typical cellulose nanocrystal (CNC) photonic films derived from biomass, known for their sustainability and cost-effectiveness, often suffer from limited sensitivity and slow response times due to their dense structure. To address this challenge, we have utilized a unidirectional interlayer freezing-photopolymerization strategy to introduce porous structures into CNC photonic films without compromising their vibrant structural color. This method harnesses ice crystal-induced lamellar pores while preserving the periodic arrangement of CNCs. The underlying mechanism of ice kinetics and CNC assembly is established, highlighting the transition from non-iridescent aerogels to iridescent, porous photonic films. The resulting porous CNC photonic film exhibits apparent color response and rapid sensing capabilities in response to various solvent stimuli, outperforming its non-porous counterparts. We have validated the film as a portable vapor detector for rapid visualized alcohol detection. This approach provides promising developments in sustainable, highly responsive sensor technologies.